Nautiloids flourished during the early Palaeozoicera, when they were the main predatory animals. They developed an extraordinary range of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, though only a few species survive today.[1]

Traditionally, the most common classification of the cephalopods has been a three-fold division into the nautiloids, ammonoids, and coleoids. This article is about nautiloids in that broad sense, sometimes called Nautiloidea sensu lato.

Cladistically speaking, nautiloids are a paraphyletic group united by shared primitive (basal) features not found in derived cephalopods. In other words, they are a evolutionary grade that is thought to have given rise to both ammonoids and coleoids. They are defined by the exclusion of both those descendent groups. Both ammonoids and coleoids probably descended from bactritids, which in turn arose from straight-shelled orthocerid nautiloids.[1]